The present invention relates to x-ray apparatus and, more particularly, to a method and apparatus for in situ evaluation of imaging apparatus in an x-ray system.
In x-ray apparatus, x-ray radiation responsive apparatus is positioned in a line with an object and an x-ray source whereby x-rays from the source pass through and are attenuated by the object before impinging on the x-ray sensitive apparatus to thereby obtain an image of the object. One type of x-ray apparatus is a "spot film" system which includes a table on which an object, e.g., a patient, is positioned and a motor driven mechanism within the table for moving the table to various desired locations. An imaging apparatus is positioned above the patient and also includes mechanisms for moving the imaging apparatus into selected positions with respect to the patient. The imaging apparatus may include a film holding mechanism for positioning sheets of x-ray film in desired locations or may include a mechanism for positioning an x-ray sensitive electronic device in position for sensing the x-rays coming through the object. The electronic device may comprise an image intensifier tube which responds to the x-ray radiation by generating an optical image having an optical intensity proportional to the intensity of the received x-rays.
In the system utilizing the electronic imaging apparatus, the optical image generated by the apparatus may be transmitted to a plurality of recording or readout devices. A recording device may include a cine film camera and video tape recorder. A read-out device may comprise simply a television camera positioned to sense the optical image and to transmit the image to a television display.
In order to present the fluoroscopic image, i.e., the optical image of the x-ray attenuation effects in an object, in a suitable form for display on the television monitor, numerous energy conversions occur. First, the electronic sensor such as, for example, the image intensifier tube, converts the incident x-ray information to a visible light image of increased brightness. An optical system, usually consisting of two very fast, highly corrected lenses, projects this image onto the pick-up tube in the television camera. This image is then converted into a television signal, transmitted over a connecting cable, and displayed on the monitor for observation. When film cameras are added into the system, more complex optical systems are required. Film cameras may include cine cameras for high speed recording of dynamic phenomena and seventy to one hundred or more millimeter cameras for use as supplementary spot film cameras.
Because of the numerous energy conversions occurring in the x-ray to optical image system, and because of the complex optics involved in transmitting the images, the imaging system becomes very susceptible to alignment variations, light attenuation or losses in optics and to variations in the quality of the image generated by the image intensifier tube.
In present day systems, once the x-ray apparatus has been placed into use, it becomes very difficult for the user to determine whether deterioration in picture or image quality is due to a failure or deterioration of components of the system and which if any of the components have deterioated. Quite often, misalignment or loss of alignment of various optical components in the system may result in deterioration of image quality. The primary suspect component in most image quality problems is the image intensifier tube. In general, due to the cost of the x-ray apparatus and the need to have the apparatus available at all times, the user will often elect to replace the image intensifier tube whenever image quality deteriorates. Quite often, tube replacement does not solve the image problem. Nevertheless, the cost and time involved in changing the tube must be incurred in order to determine whether or not the tube is the deteriorated variable. Such cost in a typical x-ray spot film apparatus is relatively high and may require an average of sixteen hours to change a tube and realign the system with a new tube in place. If such change out does not resolve the deterioration problem, further changes in the system may be required. Accordingly, it would be advantageous to provide a method and apparatus to enble quantitative and qualitative evaluation of an x-ray imaging apparatus without the necessity of a major overhaul.
In x-ray imaging apparatus utilizing both high speed cine film recording and television monitor displays, the image generated by the image intensifier tube is split between the two display and recording apparatus. The splitting of the image is accomplished by use of a beam splitter, i.e., a semi-transparent mirror which allows part of the light to pass through the mirrored surface while reflecting a portion of it towards one of the recording or display devices. In some applications, a third device such as a magnetic recording apparatus may also be utilized and in those cases the beam splitter may have to be switched between various selected positions in order to accommodate recording on the selected devices. Because of the movement of the beam splitter, it is possible that the beam splitting mirror may not return to exactly a desired location and may result in an image not being precisely positioned on one of the recording or display devices. If this were to happen, the image quality would deteriorate and the user would be unable to determine whether the deterioration was due to the beam splitter or to the image intensifier tube.
Still further, the optics in the imaging system are highly corrected lenses arranged in an infinite conjugate system. This arrangement means that light entering the lens from one of the conjugate points associated with the lens has all the rays from a single point of the target essentially parallel. In other words, in the case of an image intensifier, the output screen of the image tube is located at the focal point of the objective lens so that all the rays originating from any one point of the phosphor on the output screen emerge parallel from the lens. It will be appreciated that the lensing system includes a plurality of individual lenses precisely located with respect to each other. Any shifting of the lensing system will result in the deterioration of image quality. Furthermore, it is not unusual for the lensing system to have particular areas around the periphery such that light rays passing through the periphery are not precisely parallel and thus operate to diffuse the image and to further deteriorate image quality. For this reason, it is important to position the lensing system such that the light rays pass through that portion which provides the best image quality. However, as with the beam splitter, once the system has been set up and is being used, it becomes difficult if not impossible to determine from the generated image whether the system deterioration is caused by the image intensifier tube, the optical system or the beam splitter. Since the optical lens system and beam splitter may be aligned to correct any image deterioration, it would be advantageous to provide a method and apparatus for quantitative and qualitative evaluation of these components of an x-ray system without the necessity of having to disassemble the system and change components to determine which component is creating the image deterioration.